I had a Warwick Corvette with a dead spot in the same location as yours. It was very noticeable. Changing strings didn't help, it was the bass.I’ve got a fretless EBMM Sterling 4H strung with TI flats. I played this same combo (different Sterling) for years with no issues. My new Sterling has very noticeable lack of sustain on low D on the G string. The strings are 9 months old with maybe 100 hours on them.
Does it make sense that the same note (the octave, not open D) played on the D string is fine?
I've also noticed the same note played elsewhere on the fretboard not being dead.